1. Field of the Invention
The present invention relates to a resonant circuit and an antenna device, and more particularly, relates to a resonant circuit for a wireless IC device preferably for use in an RFID (Radio Frequency Identification) system, and an antenna device including the resonant circuit.
2. Description of the Related Art
Conventionally, as management systems for goods, RFID systems have been developed for transmitting information via non-contact communication between a reader/writer for generating electromagnetic waves and wireless ICs (referred to as IC tags, wireless IC devices, etc.) for storing predetermined information, which are attached to goods or containers. The wireless IC coupled with an antenna allows communication with the reader/writer.
This type of antenna is set to have a resonance frequency that is based on the frequency of communication signals, and basically configured to generate, together with coil-shaped conductors wound on a plane surface, a capacitance for setting the inductance of the coil-shaped conductors as well as the resonance frequency.
For example, Japanese Patent Application Laid-Open No. 2001-84463 discloses a resonant circuit with coils thick in line width and short in length, which are formed on both sides of a film, and are capacitively coupled with each other. This resonant circuit functions as an antenna, and in order to lower the resonance frequency, the coils are increased in size, because there is a need for coils with a large L value or a need to increase the C value between the respective coils. However, this has the problem of degrading communication characteristics because the size of the antenna is increased, and because the opening of the coils is reduced.